Spectral analytical unit with a diffraction grating

ABSTRACT

A spectral analytical unit for acting on a parallel light bundle having different wavelengths. The spectral analytical unit includes a diffraction grating on which the light bundle falls, the diffraction grating splitting the different wavelengths through diffraction in first spectral directions defining a light bundle diffraction order 1 without recycle, and the diffraction grating bending the light bundle in second directions defining a light bundle diffraction order 0 without recycle, a detector line made up of a plurality of elements, optics for focusing the split light bundle diffraction order 1 without recycle on the detector line, evaluation electronics connected to the detector line for obtaining data related to a created spectrum, and a deflecting device wherein the diffraction order 0 light bundle without recycle meets on the deflecting device which is so directed and positioned that this light bundle falls on the diffraction grating thereby creating a reflected diffraction order 1 light bundle with first recycle and a reflected diffraction order 0 light bundle with first recycle whereby the diffraction order 1 without recycle and the reflected diffraction order 1 light bundle with first recycle each of a part wavelength range are impressed through the optics on a single element of the detector line.

BACKGROUND OF THE INVENTION Field of the Invention

The invention concerns a spectral analytical unit with a diffraction grating in which a parallel light bundle which has a wavelength range falls on a diffraction grating which splits the different wavelengths through diffraction in first spectral directions, whereby this light bundle is named as light bundle diffraction order 1 without recycle, and the diffraction grating bends the light bundle in a second direction whereby this light bundle is named as light bundle diffraction order 0 without recycle, furthermore parts of the wavelength range of the spectrally split light bundle diffraction order 1 without recycle can be focused on a detector line (3) through optics and evaluation electronics is connected to the detector line which gets the created spectrum as information and displays. The spectral unit finds application in all spectrometers. In particular the unit comes in use in a confocal Laser Scanning Microscope (LSM), such as the one described in DE 197 02 753 A1 or U.S. Pat. No. 7,009,699 B2, as measuring equipment for the spectrally split detection of fluorescence.

The spectral unit with a diffraction grating is built in principle as a Polychromator. A broad range light radiated from a probe is broken down spectrally through a dispersive element and then is measured by means of a detection unit and evaluated. In this way, a diffraction grating is set up as a dispersive element. Principally diffraction gratings are differentiated according to levels of flat grating with equidistant lines and a picturing grating which is preferably created holographically.

With the levels flat grating, collimation optics between the grating and the light source and a focusing optics between the grating and spectrum are necessary. These optics can be lenses or mirrors. Normally the spectrum of the diffraction order 1 of the diffraction grating is created using the focusing optics on the receiver of the detection unit. The problem is that the diffraction efficiency of the diffraction grating changes very markedly depending on the wavelength, the grating constants, the grating material and the profile form i.e., the complete transmission has strong limitations because of physical conditions.

In particular with small grating constants (g) further polarization effects appear if g is of the order of magnitude of the wave length or is smaller. The polarization effects show themselves in a strong split according to the intensity of the TE and TM polarization, by which the diffraction intensity is strongly reduced at least in the border ranges of the spectrum.

With a mechanically created blazed grating it is common to determine the blaze angle of grating flanks such that for a certain desired wavelength, highest possible diffraction efficiency is reached or a compromise for the fall in diffraction efficiency is created. The bigger the spectral range, the bigger the fall.

A known method for getting around this problem is shown by the use of Echelle systems such as is described in U.S. Pat. No. 5,189,486. Here a flat grating is used in very high diffraction orders whereby at first short overlapping spectral areas come up all of which work in the neighborhood of the blaze angle. For lateral separation of the spectrum a prism is added in the system which works perpendicularly to the dispersion direction of the grating. Through this one gets many lateral overlapping order lines. However the condition for the use of this process is the use of a surface receiver.

The reason for the spectral variation of the efficiency of diffraction lies in that the electromagnetic behavior of the grating diffracts one more or less big part of the light in other than the desired diffraction order 1, through which it is lost and even further creates scattered light problems. In particular the biggest part of the energy in the 0th diffraction order 0 gets lost, particularly then when no further bigger diffraction orders 1 or no smaller 0 diffraction order can appear physically.

The invention is intended to solve the task to increase significantly the diffraction efficiency of a spectral analytical unit with a grating with small expense.

BRIEF SUMMARY OF THE INVENTION

The present invention is based on the fact that the diffraction order 0 arising in diffraction on a grating does not get “lost” energetically as for example happens through gating or absorption, but to let it couple in the spectral unit again and to diffract at least still one more time. Thus after this repeated execution there arise a part of diffraction order 1 and again a certain rest portion of diffraction order 0. This will however be coupled back and will be diffracted again. Theoretically this loop can be executed “without end” and the reached diffraction efficiency converges to the sum from the original diffraction order 0 and diffraction order 1 for the corresponding color. For the case that only diffraction order 0 and diffraction order 1 exist, one will reach ideal diffraction efficiency up to loss due to absorption over the full spectral range.

This process will be limited only through the maximum possible size of the diffraction grating, the mirror and focusing optics. In particular with moderate bundle diameters one can realize a relatively high number of cycles. The advantage of this arrangement is further that it can be relatively easily integrated retroactively in the existing Polychromator constructions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a spectral analytical unit with a diffraction grating, two deflecting mirrors and one focusing mirror.

FIG. 2 is a schematic diagram of a display arrangement according to FIG. 1 in the dispersion plane of the diffraction grating.

FIG. 3 is a schematic diagram of a section from FIG. 2 for displaying of the position of the deflecting mirror.

FIG. 4 is a schematic diagram of a side view of an arrangement according to FIG. 1.

FIG. 5 is a graphic representation showing the efficiency of the arrangement according to the invention with back coupling of the diffraction order 0.

FIG. 6 is a schematic diagram of the spectral analytical unit with a diffraction grating and one deflecting mirror.

FIG. 7 is a schematic diagram of the spectral analytical unit with one diffraction grating and three deflecting mirrors.

FIG. 8 is a schematic diagram of the spectral analytical unit with one diffraction grating, two deflecting mirrors and one collecting lens

FIG. 9 is schematic diagram of the construction of a confocal Laser Scanning Microscope with the spectral analytical unit according to the invention as a measuring device.

FIG. 10 is a schematic diagram showing a further variation of the spectral analytical unit with one diffraction grating, with two deflecting mirrors.

FIG. 11 is a schematic diagram of a spectral analytical unit in which the diffraction grating and the deflecting mirror form a monolithic unit.

DETAILED DESCRIPTION OF THE INVENTION

In describing preferred embodiments of the present invention illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the invention is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner to accomplish a similar purpose.

FIG. 1 shows schematically the construction of the Spectral Analytical Unit with one diffraction grating and two deflecting mirrors.

A falling-in, mainly parallel light bundle 10, falls on a diffraction grating 1. This will be diffracted spectrally by appropriate dimensioning of the diffraction grating 1 in a light bundle 11 diffraction order 1 and split in a light bundle 12 diffraction order 0. The light bundle 11 diffraction order 1 falls on a focusing mirror as optics 2 and is concentrated on a detector line 8 of a line receiver 3 along the expansion direction of the line. The detector line consists of individual CCD elements 7. In the expansion direction of the line the spectral split of the light bundle diffraction order 1 is detected.

To the diffraction grating 1 a first deflecting mirror 4 is so arranged that it reflects the light bundle 12 diffraction order 0 in the direction of falling-in light bundle 10. On a place which is closest possible to the falling light 10 a second deflecting mirror 5 is so positioned that it deflects the light bundle diffraction order 0 parallel to the falling-in light bundle 10 again on the diffraction grating 1. In this, the place lighted on the diffraction grating 1 from the diffraction order 0 is shifted by the amount a1 from the place of the falling-in light bundle 10 in the X direction. The light bundle 12 diffraction order 0 is diffracted there under the same conditions as the falling-in light bundle 10. The diffraction order 0 14 arising again here is freshly high shifted, through the two deflecting mirrors 4 and 5 coupled in and reaches the grating at a distance of a2. A further recycle of the light bundle 16 diffraction order 0 reaches the grating at the distance of a3. In general it is already sufficient, three such recycles, realized here with the diffraction order 0 light bundles 12, 14 and 16, to achieve a significant increase in efficiency. In the example the diffraction order 0 light bundle from the third recycle 18 is not used because it is negligible.

All the same wavelengths of the diffraction order 1 light bundle 11, 13, 15 and 17 are formed each in a corresponding point on the detector line 8 through the optics 2 so that one line on the detector line 8 reproduces the spectral characteristic. The light bundle diffraction order 1 which can be assigned to one wavelength, for example the wavelengths λ1, λ2 and λ3 are shown, meet each at one place on the detector line 8. Because the detector line 8 is made in the end from large single elements 7, in the width b of the single element 7 of about less than 1 mm a wavelength range of about 10 nm is received. In this example 32 individual receivers 7 are arranged on the detector line 8 which is 32 mm wide. The individual receivers have a height h of 10 mm. The measured wavelength range lies between 380 nm and 780 nm.

So that the process works optimally, the profile design of the diffraction grating 1 must be so made that as far as possible only diffraction order 0 occurs as false light order. This can be achieved by the maximum blaze of the grating being shifted in the short wave range. With mechanical triangular profiles this would mean that the flank angle is flatter. For example with a diffraction grating which at 500 nm diffracts 70% in diffraction order 1 and the remaining 30% in the diffraction order 0, the following behavior appears:

-   -   Intensity after 1st cycle=70% (starting status) (30% remains in         the diffraction order 0)     -   Intensity after 2nd cycle=70%+70%×30%=91% (9% remains in the         diffraction order 0)     -   Intensity after 3rd cycle=91%+70%×9%=97.3% (2.7% remains in the         diffraction order 0).

With this example calculation the absorption losses are not taken into consideration because these are negligible.

FIG. 2 is a display of the arrangement according to FIG. 1 in a display in which the falling-in light bundle 10 and the diffracted light bundles 11, 13, 15, 17 of the diffraction order 1 are in the plane of the paper. The 0 diffraction order light bundles 12, 14 and 16 are diverted through the first deflecting mirror 4 under the falling-in light bundle 10. The second deflecting mirror 5 diverts this 0 order diffraction light bundle parallel again on the diffraction grating 1 to the light bundle diffraction order 1.

FIG. 3 shows a section from the FIG. 2 drawing to illustrate the position of the axes of the reference system. The coordination origin lies in the apex of the diffraction grating 1, the positive z axis points in the direction of the extended falling-in light bundle 10. The series of the translation or position of the construction element is as follows:

Details of the translation in directions x, y, z

Rotation around the corresponding surface particular X axis (alpha angle)

Rotation around the corresponding surface particular Y axis (beta angle)

Rotation around the corresponding surface particular Z axis (gamma angle)

Construction Element X Y Z alpha beta gamma Origin 0.0 0.0 0.0 0.0 0.0 0.0 Diffraction grating 1 (line grating) 0.0 0.0 0.0 36.0 0.0 0.0 Deflecting mirror 4 (plane mirror) 0.0 −38.04226 −12.36068 99.0 −3.4006 0 Deflecting mirror 5(plane mirror) 5.00273 0.03365 −40.14952 153.1634 −3.3957 4.1346 Optics 2 (focusing mirror) 1.08218 −40.56915 −74.59381 −176.3830 0.6787 5.1892 Line receiver 3 (CCD line) −0.82344 −65.81362 −8.70134 −178.0008 0.5315 5.2063

The invention makes it possible to detect a spectral range from 380 nm-700 nm. In this the length of the spectrum is 31.2 mm. The diameter of the falling-in light bundle 10 is 3.0 mm and the grating density is ascertained at 1300 lines/mm. The offset of the 0 diffraction order light bundle in X direction comes to 5.00 mm per cycle. The first deflecting mirror 4 and the second deflecting mirror 5 are plane mirrors, the optics 2 is a mirror with a radius of 151.0 mm (cc).

FIG. 4 shows a further view of the arrangement shown in FIG. 1 in the X-Z plane. The diagram plane lies perpendicular to the dispersion plane. The diffraction order 0 light bundles 12, 14, 16 fall in the X direction shifted on different places on the diffraction grating 1.

FIG. 5 shows the diffraction efficiency as a function of the wavelength for a known simple arrangement with a line grating with 1302 lines in Aluminum. The diffraction order 1 light bundle 11, which is created from a falling-in light bundle, reaches in the spectral region about 530 nm its maximum intensity at nearly 80% and falls continuously on both sides up to nearly 40%. The corresponding curve is identified in the graph as 11.

Further the Figure shows that with an increasing number of back couplings of the 0 diffraction order light bundle an increasing efficiency increase is achieved. To the intensity of diffraction order 1 light bundle 11 from the falling-in light bundle are added the intensities of the 1 diffraction order light bundle 13 from the first recycle as well as the intensities of light bundle 15 and 17 from further recycles. The corresponding resulting curves are identified in the graph as 11, 13, 15 and 17.

FIG. 6 shows schematically a Spectral Analytical Unit with one diffraction grating and one single deflecting mirror. The falling-in light bundle 10 falls here slightly tipped around the y axis on the diffraction grating 1 whereby the diffraction grating 1 and the deflecting mirror 4 stand preferably parallel to each other. Here the diffraction order 0 light bundles are reflected back on the diffraction grating 1 with the deflecting mirror 4 under the plane of the diagram.

FIG. 7 shows schematically a Spectral Analytical Unit with one diffraction grating and three deflecting mirrors 4, 5 and 6. But more than three mirrors can also be used.

FIG. 8 shows a Spectral Analytical Unit corresponding to FIG. 1 with one diffraction grating 1, two deflecting mirrors 4,5, one collecting lens as optic 2 and line receiver 3.

FIG. 9 shows the schematic construction of a confocal Laser Scanning Microscope 101 with the spectral analytical unit 100 as measuring device. The spectral analytical unit 100 corresponds in its construction to the arrangement shown in FIG. 7. The radiation bundle going out from the light source 20 reaches through a main color splitter 21, an x-y scanner 22, a scan optics 23, a tubular lens 25 and a lens 26 to the probe 27.

The light bundle going out from the probe 27 reaches through the lens 26, the tubular lens 25, the scan optics 23, the x-y scanner 22, the main color splitter 21 as well as a pinhole optics 28, a pinhole 29, a collimator optics 30 and an emission filter 31 to the diffraction grating 1.

Between the scan optics 23 and the tubular lens 25 arises an intermediate image 24. With the Spectral Analytical Unit according to the invention an increase of up to more than 40% in the light yield with a spectral measurement is achieved with comparatively smaller expense. Thereby the space requirement for the additional deflecting facility and the additional radiation flow is small. Particularly advantageous is also that the invention can be built in already available Laser Scanning Microscopes and other spectrometric devices.

According to FIG. 10, it is also possible to rotate the second deflecting mirror 5 in such a way that the 0 diffraction order light bundles 12, 14, 16 meet the diffraction grating 1 at the same place after each cycle. However in this case arises an offplane angle opposite to the direction of dispersion at grating through which the spectral focal points for the individual cycles stand in different lateral heights h on the line receiver 3. With the given adequate size receiver heights h wavelength part areas of all spectrally split light bundles 11, 13, 15, 17 of diffraction order 1 from the different cycles are caught by each individual element 7.

FIG. 11 shows a Spectral Analytical Unit, with which the diffraction grating 1 and the deflection device build a unit. A prism shaped basic body (prism section 40) contains a light entry surface for the falling-in light bundle 10 and a light exit surface 44 for the diffraction order 1 light bundles 11, 13, 15, 17. Two side walls are applied reflective coating and build with the sealing surfaces 41 and 42 deflecting device. The diffraction grating 1 is placed in a ground surface of the prism section 40.

REFERENCE DIAGRAM LIST No. Description

-   1 Diffraction grating 1 (line grating) -   2 Optics (focusing mirror, lens) -   3 Line receiver (sensor elements of a CCD) -   4 First deflecting mirror (flat mirror) -   5 Second deflecting mirror (flat mirror) -   6 Third deflecting mirror (flat mirror) -   7 Individual element -   8 Detector line -   9 Evaluation electronics -   10 Falling-in light bundle (parallel) -   11 Light bundle—diffraction order 1 without recycle (from the     falling-in light bundle) -   12 Light bundle—diffraction order 0 without recycle (from the     falling-in light bundle) -   13 Light bundle—diffraction order 1 from the first recycle -   14 Light bundle—diffraction order 0 from the first recycle -   15 Light bundle—diffraction order 1 from the second recycle -   16 Light bundle—diffraction order 0 from the second recycle -   17 Light bundle—diffraction order 1 from the third recycle -   18 Light bundle—diffraction order 0 from the third recycle is     negligible -   19 -   20 Light source -   21 Main color splitter -   22 X-Y scanner -   23 Scan optics -   24 Intermediate image

Tubular Lens

-   25 Lens -   26 Probe -   27 Pinhole optics -   28 Pinhole -   30 Collimator optics -   31 Emission filter -   40 Prism section -   41 Light entry surface -   42 Mirror surface -   43 Mirror surface -   44 Light exit surface -   100 Spectral Analytical Unit -   101 Confocal Laser Scanning Microscope -   a Distance of diffraction order 0 light bundle from the falling-in     light bundle -   α Angle of diffraction order 0 relative to the falling-in light     bundle -   h Height of the individual receivers, height of the detector line -   b Width of the individual receiver -   d Width of the detector line -   n Number of individual receivers

It is to be understood that the present invention is not limited to the illustrated embodiments described herein. Modifications and variations of the above-described embodiments of the present invention are possible, as appreciated by those skilled in the art in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims and their equivalents, the invention may be practiced otherwise than as specifically described. 

1. A spectral analytical unit for acting on a parallel light bundle having different wavelengths, the spectral analytical unit comprising: a diffraction grating on which said light bundle falls, the diffraction grating splitting the different wavelengths through diffraction in first spectral directions defining a light bundle diffraction order 1 without recycle, and the diffraction grating bending the light bundle in second directions defining a light bundle diffraction order 0 without recycle, a detector line made up of a plurality of elements, first optic means for focusing the split light bundle diffraction order 1 without recycle on the detector line, evaluation electronics connected to the detector line for obtaining data related to a created spectrum, and a deflecting device wherein the diffraction order 0 light bundle without recycle meets on the deflecting device which is so directed and positioned that this light bundle falls on the diffraction grating thereby creating a reflected diffraction order 1 light bundle with first recycle and a reflected diffraction order 0 light bundle with first recycle whereby the diffraction order 1 without recycle and the reflected diffraction order 1 light bundle with first recycle each of a part wavelength range are impressed through the first optic means on a single element of the detector line.
 2. The spectral analytical unit according to claim 1, wherein the reflected diffraction order 0 falls on the diffraction grating under the same angle as the falling-in light bundle, with the reflected diffraction order 0 being shifted at a distance (a) in X direction to the falling-in light bundle.
 3. The spectral analytical unit according to claim 1, characterized in that the reflected diffraction order 0 light bundle falls on the diffraction grating under a different angle α, but on the same place as the falling-in light bundle.
 4. The spectral analytical unit according to claim 1, wherein the reflected diffraction order 0 light bundle from the first recycle meets on the deflecting device, further falls on the diffraction grating and a diffraction order 1 light bundle from a second recycle and a diffraction order 0 light bundle from a second recycle are created whereby the diffraction order 1 light bundle without recycle, the diffraction order 1 light bundle from the first recycle and the diffraction order 1 light bundle from the second recycle of corresponding part wavelength range are impressed through the first optic means on a single element of the detector line.
 5. The spectral analytical unit according to claim 4, wherein the diffraction order 0 light bundle from the second recycle and the diffraction order 0 light bundle from at least one further recycle meet on the deflecting device, and are reflected on the diffraction grating and are spectrally split.
 6. The spectral analytical unit with a diffraction grating according to claim 5, wherein the reflected diffraction order 0 light bundles fall on the diffraction grating under the same angle as the falling-in light bundle, however the reflected diffraction order 0 light bundles are shifted at a distance (a₁, a₂, a₃) in X direction to the falling-in light bundle.
 7. The spectral analytic unit according to claim 5, wherein a plurality of the reflected diffraction order 0 light bundles fall on the diffraction grating under a deviating angle (α₁, α₂, α₃), on the same place as the falling-in light bundle, so that all diffraction order 1 light bundles of a part wavelength range are impressed in a direction perpendicular to the width (d) of the detector line on the detector line and individually shifted, whereby the height (h) of a single element of the detector line is so large that all diffraction order 1 light bundles of the part of the wavelength range are detectable.
 8. The spectral analytic unit according to claim 1, wherein the deflecting device comprises: a first deflecting mirror, which reflects the diffraction order 0 light bundles diffracted from the diffraction grating and back to the diffraction grating again after one reflection.
 9. The spectral analytic unit according to claim 1, wherein the deflecting device comprises a first deflecting mirror and a second deflecting mirror, which together reflect the diffraction order 0 light bundles diffracted from the diffraction grating and back to the diffraction grating again after two reflections.
 10. The spectral analytic unit with a diffraction grating according to claim 1, wherein the deflecting device comprises a combination of at least first and second deflecting mirrors.
 11. The spectral analytic unit according to claim 1, wherein the deflecting device comprises a prism section which has a minimum of first through fourth optically effective surfaces perpendicular to a dispersion plane of the diffraction grating, wherein the first optically effective surface constitutes one light entry surface that lies opposite to the diffraction grating with the diffraction grating being placed in the prism section, the second optically effective surface constitutes a light exit surface for the diffraction order 1 light bundle and a third optically effective surface constitutes a first mirroring surface for reflection of the diffraction order 0 light bundle.
 12. The spectral analytic unit according to claim 1, wherein the fourth optically effective surface constitutes a second mirroring surface for reflection of the diffraction order 0 light bundle.
 13. The spectral analytic unit according to claim 1, wherein the first optic means comprises a focusing mirror.
 14. The spectral analytic unit according to claim 1, wherein the first optic means comprises a light bundling lens. 